Search for Anomalous Production of Multi-
lepton Events at CDF
Alon Attal UCLASept. 5, 2006
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Outline
Standard Model
SUSY Theory & Motivation
CDF Detector
Signal & Background
Analysis
Results
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Standard Model
The Standard Model (SM): SU(3) x SU(2) x U(1)Y
The SM agrees extraordinarily well with detector measurements
SM requires a Higgs boson to explain massive particles
Is there physics beyond the SM?
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New Physics Clues
SM does not contain gravity New physics at Planck Scale (~1018 GeV)
Higgs has not been discovered yet Could be an indication of non-SM Higgs
Hierarchy problem Unnatural for Higgs mass corrections to depend on Planck Scale
Dark Matter None of the particles within SM can account for it
Particle – antiparticle asymmetry Can’t be accounted for with SM CP violation
Particle masses, quantized electric charge, 3 families SM doesn’t answer why it is the way it is
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What is SuperSymmetry?SuperSymmetry (SUSY) is a proposed symmetry between fermions
and bosons:Fermions Bosons
SM particles
SM particles/Higgs
SUSY particles
SUSY particles
SUSY must be broken particles gain mass
SUSY contains particles exactly the same as Standard Model particles but with different spin:
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SUSY Motivation
SUSY cancels quadratic divergences in the Higgs sector:
Works best when particle, SM masses are similar
particles should have masses on the order of Mtop
-
f
f
H
f~
f~
H H H≈ 0
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SM
(GeV)
Grand Unification
Addition of particles leads to unification at GUT Scale
SUSYi-1
()
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Minimal Supergravity (mSUGRA) SUSY introduces > 100 free parameters
The mSUGRA framework constrains the theory down to 5: M0,
M1/2, tan,A0, sign(). In my analysis M( ) is the most important and depends on M1/2
0~1
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R-Parity (Rp)
Rp = (-1)[3(B-L) + 2S]
+1 for SM particles
-1 for particles
B = baryon #, L = lepton #, S = spin
If Rp is conserved, the lightest supersymmetric particle (LSP) is stable.
LSP = Dark Matter candidate
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R-Parity Violation (RpV)
“RpV SUSY is no WIMP”
Violate lepton # Violates baryon #
3 additional couplings:
• Only consider Li Lj Ek term, protecting proton lifetime• |λ| < 0.1, assume only LSP decays via RpV coupling
WRPV ijk LiL jE k ijk LiQ jD k ijkU iD jD k
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4-Lepton SignatureStep 1: Dominant Sparticle Production
2 particles
p p
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Step 2: Chargino/Neutralino Decay
2 particles
2 LSPs
cascade decay
p p Leptons may be produced in cascade decays.
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Step 3: RpV Decays
121 diagrams:
2 particles
2 LSPs
≥ 4 Charged Leptons
cascade decay
RpV decay
p p Most sensitive to 121 and 122.
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Restrictions on There are theoretical and experimental limits on :
Upper limit set by muon lifetime
Analysis is not sensitive to the specific value of since
Lower limit set by impact parameter cut on identified leptons (d0 ≤ 0.02 cm)
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Tevatron & Luminosity
Currently the highest energy accelerator in the world
346 pb-1 used
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CDF Detector & Lepton ID
Lepton ID important to analysis Studied in data and MC Efficient (~90%) Probability that jets are misidentified
as leptons is small (< 0.02%)Muon Detectors
Drift ChamberCalorimeter
Electrons Track + Calorimeter Cluster 95% of energy in EM calorimeter |η| < 2.0
Muons Track plus “stub” in muon detector Minimum ionizing |η| < 1.1
η = 1.0
η = 2.0
η = 0
~
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Signal Distributions
muon coverage
electron coverage
181.4
M
(GeV)
0.13
(pb)
0
A0
+
sign
182.299.45260250
M
(GeV)
M
(GeV)
tanβM1/2
(GeV)
M0
(GeV)
±~1
0~2
0~1
Analysis Reference Point:
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Needle in a Haystack
SUSY σ much smaller than backgrounds
Key: understand and reduce backgrounds
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Data Collection
All Events
High-pT lepton triggers:Electron or muon with pT > 18 GeV/c
Triggered Events
Filtered Events
Dilepton Filter:“Tight” lepton w/ pT > 18 GeV/c and
“loose” lepton w/ pT > 5 GeV/c
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Backgrounds (Before Final Cuts)
Luminosity = 346 pb-1
Individual processes t-tbar / heavy flavor Diboson
Composite backgrounds W / Z/* + Misidentified
Jets – Determined from data
W / Z/* + Photon Conversions (+ material → e+e-)– Determined from MC
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Analysis Strategy
Counting experiment with 346 pb-1
No Jet or missing energy cuts to reduce model dependency
Also sensitive to H±± and non-minimal SUSY models Two signal regions “boxes” to optimize result:
4 or more leptons Exactly 3 leptons (to increase acceptance)
Choose selection criteria to optimize S/B Validate with control regions
Goal: Find new physics in multi-lepton channel!
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Selection Criteria & Targeted Backgrounds
Di-Boson
Heavy Flavor
Drell-Yan
Z/ + Jets
Trilepton Signal Regions: At least one of the two leading leptons must be an e± (±) for 121 (122)
ET (pT) > 20, 8, 5 GeV/c2 (GeV/c)
for 1st, 2nd, additional leptons, respectively
Electron track + opp. sign partner track:
S < 0.1 cm, cot < 0.02
Low Mass Cut: Mll < 15 GeV/c2
Isolated Leptons
Z Veto Cut: ≠ (76 < Ml+l- < 106 GeV/c2)
Cut: ≠ ( 160 < < 200°)
, J/Selection Requirement
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Systematic Uncertainties
Lepton ID Per lepton (0.7 – 11% depending on lepton type, pT)
Mis-ID Jets 50%
Conversion Removal 28.8%
Luminosity 6%
PDFs 0.3% (signal), 2% (background)
ISR 1.5% (signal), 4% (background)
Cross Section 8% (DY), 5% (Diboson), 10% (t-tbar), 7% (signal)
Understanding uncertainties important for reporting the accuracy of the result
Largest systematic uncertainties: Mis-ID Jets for background expectation Lepton ID for signal expectation
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Control Region (Example) Control regions are crucial to understanding our procedure
Validate lepton ID efficiencies Validate selection cuts
Trilepton events that fail 1 or more cuts
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Control Region Overview
y = x
26 total control regions (summary on left) By lepton type Inside & outside Z window Number of leptons Pass/Fail cut
Analysis procedure is validated through agreement between data and MC prediction
Each point corresponds to a singlecontrol region w/ error bars = ±1σ
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Opening the Box
?
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Signal Regions
Signal events: 4 eee, 1ee, 1 e Probability of observing ≥ 5 events with 3.1 expected = 17% Signal regions consistent with background and no signal
Trilepton Signal Regions
Dataset λ121 λ122
Z/ + 2.1 ± 0.8 1.2 ± 1.0
Z/ + W 0.2 ± 0.1 0.1 ± 0.1
Fakes 0.7 ± 0.4 0.5 ± 0.3
Total Background 3.1 ± 0.9 1.9 ± 1.0
RpV SUSY (121) 3.8 ± 0.4 -----
RpV SUSY (122) ----- 4.0 ± 0.4
Data 5 1
≥ 4 Lepton Signal Region
Dataset Signal
Z/ + 0.001 ± 0.001
Z/+Z/ 0.004 ± 0.002
Fakes 0.004 ± 0.003
Total Background 0.008 ± 0.004
RpV SUSY (121) 1.5 ± 0.2
RpV SUSY (122) 1.5 ± 0.3
Data 0
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Setting Limits
Use Bayesian method to find σobs, combining 3 and ≥4 lepton signal regions
Set limits for multiple SUSY scenarios
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Results
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Trilepton Event
Jet 16 GeV
Leading Mee = 70 GeV/c2
e- 50 GeV, e+ 30 GeV, e- 13 GeV
e-50 GeV
e+30 GeV
e-13 GeV
e+30 GeV
e-50 GeV
e-13 GeV
Jet 16 GeV
Jet 16 GeV
ET = 1.5 GeV
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Trilepton Invariant Mass
TrileptonSignal Events
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DØ RpV SUSY Search
DØ searched for same process but in 3l + ET channel
Disadvantages: Sensitive to less areas
of new physics Overlap with Rp
conserving search
Advantages: Missing energy cut
reduces Z/* background
Better limits
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Rp Conserving SUSY Searches CDF and DØ both have conducted searches for associated
production of chargino-neutralino events Results combine 3l + ET and like l±l± ET searches
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Comparing Limits
RpV mass limits > RpC mass limits Small branching ratio into leptons El from LSP decay > El from cascade decays
DØ RpC mass limits > CDF mass limits Expected limits are similar DØ assumes no slepton mixing
103450ViolatedLEP
229-2343603l + ETViolatedDØ
186-203350≥ 3lViolatedCDF
140300-1,1003l + ETConservedDØ
127310-7503l + ETConservedCDF
M( ) Limit (GeV/c2)
Luminosity (pb-1)
SignatureRpExperiment ±~1
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Conclusions
We completed a search for new physics in the multilepton channel
No significant evidence of physics beyond the SM was detected
We set mass limits on the lightest neutralino and chargino using an RpV SUSY framework
1st draft approved by internal review committee and sent to collaboration for review on 8/2/06
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Backup
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particle Masses
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mSUGRA Parameter Dependence
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Electron ID CutsVariable Tight Cuts (TCE) Loose Cuts (LCE)
Fiducial fidEle == 1 fidEle == 1
ET ≥ 5 GeV ≥ 5 GeV
Track pT ≥ 4 GeV/c ≥ 4 GeV/c
Track |Z0| < 60 cm < 60 cm
# COT Ax. Seg. ≥ 3 (5 hits) ≥ 3 (5 hits)
# COT St. Seg. ≥ 2 (5 hits) ≥ 2 (5 hits)
Had/Em < 0.055 + 0.00045*E
< 0.055 + 0.00045*E
Track |d0| < 0.02cm or < 0.2 cm (No SVX hits)
< 0.02 cm or < 0.2 cm (No SVX hits)
Iso/ET < 0.1 < 0.1
E/p < 2 or pT>50 GeV -----
Lshr < 0.2 -----
Q*X -3 – 1.5 cm -----
|Z| < 3 cm -----
2strip < 10 -----
Variable Phoenix Plug
Stand-Alone
1.2 – 2.0 1.2 – 2.0
ET > 15 GeV 5-15 GeV
Track Phoenix Pad Track
Had/Em < 0.05 < 0.05
PES 5/9 (U &V) > 0.65 > 0.65
2PEM < 10 < 10
Iso/ET < 0.1 < 0.1
E/p ----- < 3
Track |d0| < 0.02cm or < 0.2 cm (No SVX hits)
< 0.02 cm or < 0.2 cm (No SVX hits)
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Muon ID CutsVariable CMUP CMX CMIO
Stub CMU & CMP CMX -----
Track Fiducial CMU & CMP CMX ≠ (CMU & CMP) and ≠ CMX
CES fiducial (2nd lepton only)
pT > 5 GeV > 5 GeV > 10 GeV
Track |Z0| < 60 cm < 60 cm < 60 cm
Em + Had Energy > 0.1 GeV > 0.1 GeV > 0.1 GeV
Em Energy < max[2 , 2 + (pT -100) * 0.0115] GeV < 2 GeV
Had Energy < 3.5 + pT / 8 GeV (pT < 20 GeV)
< max[6 , 6 + (pT -100) * 0.028] GeV (pT > 20 GeV)
< 3.5 + pT / 8 GeV (pT < 20 GeV)
< 6 GeV (pT > 20 GeV)
Track |d0| < 0.02 cm or < 0.2 cm (No SVX hits or pT < 20 GeV)
# COT Ax. Seg. ≥ 3 (5 hits) ≥ 3 (5 hits) ≥ 3 (5 hits)
# COT St. Seg. ≥ 2 (5 hits) ≥ 2 (5 hits) ≥ 3 (5 hits)
Iso/pT < 0.1 < 0.1 < 0.1
CMU Stub Cuts |X| < 5 cm or
(2 < 9 and pT < 20 GeV)
CMX Stub Cuts |X| < 6 cm or
(2 < 9 and pT < 20 GeV)
Exit Radius > 140 cm (1st lepton only)
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Additional Control RegionsExactly 2 opp. Sign electrons
Z MassWindow
Trilepton events that fail 1 or more cuts
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Control Region Overview
26 total control regions (summary on left) By lepton type Inside & outside Z window Number of leptons Pass/Fail cut
Analysis procedure is validated through agreement between data and MC prediction
StMu = Stubbed Muon, X = electron or muon
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Limits
Limits
SUSY
Scenario
M
(GeV/c2)
M
(GeV/c2)
expected observed expected observed
> 0 105.0 101.5 191.9 185.3
< 0 101.1 97.7 192.2 185.6
> 0 107.7 110.4 197.5 202.7
< 0 102.7 106.3 195.3 201.9
0~1
±~1
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DØ Limits
Limits
SUSY
Scenario
M
(GeV/c2)
M
(GeV/c2)
> 0 118 229
< 0 115 230
> 0 119 231
< 0 117 234
> 0 86 166
0~1
±~1
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Trilepton Event 2
e-
e+
Jet #1
Jet #1
Jet #3
Jet #2
e-e+
-
-ET = 2.2 GeV